The present invention relates to a current limiter and in particular to a resistive superconducting current limiter.
Current limiters prevent unacceptable large current surges in high power systems during power fluctuations, lightning strikes and short circuits and thus protect expensive electrical equipment from damage. The need for current limiters is associated with the continuous growth and interconnection of modern power systems which results in a progressive increase of short circuits to levels far beyond the original design capacity of the switchgear.
Current limiters can be grouped as resistive, inductive or hybrid, which operate by changing their impedance from nearly zero during normal operations to a current limiting value during fault conditions. The ideal performance characteristics of a fault current limiter include; zero impedance under normal operating conditions, high impedance under fault limiting conditions, fast transition from normal to fault limiting conditions, fast recovery to normal protection after interruption of a fault, high reliability over long periods with minimal maintenance, low volume, low weight and low cost.
Superconductors, high temperature (HTC) or low temperature (LTC), offer attractive potential as fault current limiters due to the great contrast between the superconducting and non-superconducting states.
The application of low temperature metallic superconductors to power engineering is limited by their low operational temperature which required liquid helium refrigeration for large scale devices. The cryogenic engineering of liquid helium is sophisticated, costly and demands specialised technical support.
In contrast high temperature ceramic superconductors remain superconducting at transition temperatures above 77 K, which is the saturation temperature of liquid nitrogen at one atmosphere. The low cost, reliability and simplicity of refrigeration at liquid nitrogen temperatures makes high temperature superconducting materials very attractive to the power engineering industry.
High temperature superconductors are however inhomogenous, anistropic and brittle materials. Their use as fault current limiter encounters problems of local heating and mechanical failure when a fault current is applied.
Resistive superconducting fault current limiters incorporate a superconducting element connected in series with the system to be protected. When the system is carrying normal operating current the element is in the superconducting state and thus has near zero resistance. The element is driven to the resistive state when a system fault occurs. The increase in current exceeds the critical current of the superconducting material which quenches to a resistive state. The impedance of the device (which is predominately resistive) increases rapidly providing the fault limiting effect.
A problem with resistive superconducting fault limiters occurs if part of the superconducting element becomes resistive before the rest of the element. This is known as xe2x80x9clocal quenchingxe2x80x9d and is due to a non-uniformity of superconducting properties along the element length. The quenched part of the superconducting element overheats and may burn out leading to a catastrophic failure.
To avoid the problem of local quenching it is known to use triggering techniques to obtain a fast and uniform transition of the superconductor to the resistive state. Known triggering techniques include laser heating to exceed the critical temperature of the superconductor, discharging capacitors to exceed the critical current density of the superconductor and external sources of magnetic field to exceed the critical flux density of the superconductor. The triggering technique has to respond within a few milliseconds in order to limit the current before it reaches its peak value and must provide sufficient energy to quench the whole of the superconductor.
British patent number 1,236,082 discloses a resistive superconducting fault current limiter which uses a low temperature metallic superconductor. In this patent a magnetic field produced by a helmholtz coil is used to quench the low temperature superconductor. A problem with this fault current limiter is that the magnetic field is radial and introduces an external Lorentz force on the superconducting elements. This arrangement is therefore unsuitable for use with high temperature superconductors as the Lorentz forces generated by the radial field would cause mechanical failure of the brittle high temperature superconductor elements.
The present invention seeks to provide a triggering technique suitable for use with high temperature superconductors. A fault current limiter in accordance with the present invention solves the problem of local heating and mechanical fracture failure in superconductive elements.
High temperature superconductors offer an attractive cost effective design due to the reduced cooling costs compared to low temperature metallic superconductors. The superconductive fault current limiter in accordance with the present invention has flexible design features to satisfy operating specifications by easily varying the physical, electrical and magnetic parameters. Uniform quenching of the superconductor is achieved by the combination of the critical current density and critical magnetic field intensity. The design has the advantage of being light weight, compact, high impedance ratio and can be easily upgraded to a higher rating fault current limiter.
According to the present invention a resistive superconductive current limiter comprises a superconducting element maintained in a superconducting state when carrying an electrical current under normal operating conditions, and means for generating a magnetic field parallel to the superconducting element so that when a fault occurs the increase in the electrical current through the means for generating the magnetic field causes the magnetic field to be generated parallel to the superconducting element, the magnetic field generated exceeds the critical magnetic field density of the superconducting element to assist triggering of the transition of the superconducting element to a resistive state.
The application of a magnetic field parallel to the superconducting elements offers the advantage that no external forces are introduced. Mechanical failure of the superconducting elements does not therefore occur.
Preferably the magnetic field is uniform which prevents local quenching which can cause thermal and mechanical failures due to local heating in high temperature superconductors. The application of a uniform magnetic field is also used to increase the rate of change of resistance of the superconductor, which in effect helps to produce a fast response fault current limiter.
The uniform magnetic field parallel to the superconductor is generated by a winding through which an electrical current passes and which is preferably connected in series with the superconducting element.
The winding is preferably a wound conductor foil of for example copper or aluminium. The winding is preferably connected in series with the superconducting element so that when a fault occurs the increase in electrical current through the winding causes a magnetic field to be generated which exceeds the critical magnetic field of the superconducting element.
Foil windings are simple to produce and self-supporting. The production of the foil windings can be automated to produce a cost effective fault current limiter system.
In the preferred embodiment of the present invention the superconducting element is located in a cryostat which is filled with a fluid at a temperature low enough to maintain it in the superconducting state. The cryostat is non-metallic and is placed within the winding.
The superconducting element may be a non-inductively arranged set of resistive elements.
Preferably means, such as a reactor or resistor, is connected in parallel with the fault current limiter to limit the transient overvoltage in the superconducting element when in the resistive state.